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December 2007, to be published in Physical Review D1
Are there approximate relations among transverse momentum dependent distribution
functions?
Certain {\sl exact} relations among transverse momentum dependent parton distribution functions due to QCD equations of motion turn into {\sl approximate} ones upon the neglect of pure twist-3 terms. On the basis of available data from HERMES we test the practical usefulness of one such ``Wandzura-Wilczek-type approximation'', namely of that connecting $h_{1L}^{\perp(1)a}(x)$ to $h_L^a(x)$, and discuss how it can be further tested by future CLAS and COMPASS data. © 2008 The American Physical Society.
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